1. Field of the Invention
The present invention relates to a method and to a device for determining the lambda value with a broadband lambda sensor of an internal combustion engine, e.g., of a motor vehicle, and a computer program that carries out all steps of the method according to the present invention.
2. Description of the Related Art
A broadband lambda sensor is known for example from German patent document DE 102 16 724 C1. The sensor is used to determine the oxygen concentration in the exhaust gases of internal combustion engines in order to obtain a control signal for adjusting a fuel-air mixture with which the internal combustion engine is operated. Such a lambda sensor has a measurement, or Nernst, cell, having a measurement electrode and a reference electrode that are situated on a solid electrolyte, and has a pump cell having an outer electrode and an inner electrode (also called the inner pump electrode). The inner electrode of the pump cell is situated, together with the measurement electrode of the Nernst cell, in a measurement chamber that is connected to the exhaust gas of the internal combustion engine via a diffusion barrier. The outer electrode is covered with a porous protective layer and is exposed directly to the exhaust gas.
In order to operate the lambda sensor, this sensor is connected to a control device that generates control signals in order to adjust the fuel-air mixture in the internal combustion engine. The Nernst cell is connected to the control device via clamps that belong to the measurement electrode and to the reference electrode. Between these clamps, a Nernst voltage can be picked off that is a function of the ratio of the oxygen concentrations to which the measurement electrode and the reference electrode are exposed. The pump cell is connected to the control device via clamps belonging to the inner electrode and outer electrode, at which clamps a pump voltage is present. The control device has a regulating circuit with which the pump voltage is adjusted as a function of the Nernst voltage.
A method and a device for controlling an internal combustion engine using such a lambda sensor is known for example from published German patent application document DE-OS 42 08 002. Here, from an acquired lambda value (ratio of air to fuel) and from a signal relating to the supplied air mass, the actually injected quantity of fuel is calculated, which is then compared to a signal relating to the quantity of fuel that is to be injected. Based on the difference between these two values, a pump characteristic field of an injection pump is adapted or corrected. With the pump characteristic field corrected in this way, the quantity of fuel to be injected corresponds to the actually injected quantity of fuel, and thus provides a very precise load signal, e.g. a load signal required for an exhaust gas recirculation.
In single cylinder injection systems, instead of the named pump characteristic field a control duration characteristic field for the injection valves is provided. In these fields, as a function of the quantity of fuel to be injected, it is stored how long the injection valves are charged with an electrical signal in order to achieve a particular opening duration or injection duration. In addition, for example from published German patent application document DE 103 31 159 A1, it is known that using the calculated deviation between the fuel quantity to be injected and the fuel quantity actually injected, it is also possible, or alternatively possible, to manipulate the air quantity in order to correct the fuel-air mixture.
In addition, from published German patent application document DE 198 31 748 A1, for the determination of the actual fresh air mass it is known not only to acquire the mass flow of the humid fresh air using an air mass sensor, but to correct this by the water vapor mass flow, because in the case of humid fresh air the same air mass flow supplies a lower oxygen mass flow for the combustion than in the case of dry fresh air. Moreover, the air-fuel ratio necessary for an optimal combustion is also a function of the air humidity, because given high air humidity a part of the effect of the exhaust gas recirculation is already anticipated, so that given high air humidity a higher lambda value is to be regarded as advantageous than in the case of low air humidity. This is based in particular on the fact that due to the air humidity the temperature in the combustion chamber is reduced. Due to the humidity-dependent oxygen content of fresh air, and the influence of humidity on the combustion temperature, in published German patent application document DE 198 31 748 A1 it is also provided to select the target value of the fresh air mass as a function of the air humidity.
According to published German patent application document DE 198 31 748 A1, the humidity content of the air is acquired by a humidity sensor, and on the basis of the acquired humidity an air mass correction is carried out of the actual fresh air mass, a lambda correction is carried out, and the target value calculation for the fresh air mass is carried out. In the air mass correction of the actual fresh air mass, measurement errors of the air mass sensor caused by the presence of the water vapor are also taken into account.
As is also known from published German patent application document DE 10 2005 050 269 A1, the conversion of the measured oxygen concentration into the named air-fuel mixture ratio (corresponding to the lambda value) standardly takes place according to the “Pischinger equation” (equation (1)) described below. In published German patent application documents DE-OS 42 08 002 and DE 198 31 748 A1, the resulting lambda value is then used in a fuel quantity calculation in order to carry out the above-named adaptation of the pump characteristic field of an injection pump.
The measurement current supplied by the lambda sensor is proportional to the oxygen content or rich gas content in the exhaust gas. Due to manufacture-related scatter, each such sensor has an individual proportionality factor. In order nonetheless to obtain the same characteristic curve for all sensors, i.e. the same measurement current for a given exhaust gas composition, an individual comparison can be provided in the sensor manufacturing process, e.g. by an electrical comparison resistor. Due to aging effects and environmental influences, the proportionality factor can additionally change. The deviation of the proportionality factor of a sensor (with or without comparison in the sensor manufacturing) from a nominal value is therefore standardly and predominantly determined, in diesel engines, using the so-called “thrust comparison.” Here the fact is exploited that, in thrust switch-off phases of the fuel supply, the internal combustion engine flushes the lambda sensor with air, and, for this gas composition which is thereby known, determines the error of the proportionality factor. The ratio between the nominal and actual proportionality factor is stored as the “comparison factor,” and is used for the multiplicative correction of the variable that characterizes the O2 concentration.
In addition, it is known that the air humidity has an influence on the volume concentration, or the molar portion of O2, in fresh air. This property of atmospheric air results from Dalton's Law, according to which the air pressure is equal to the sum of all partial pressures of the individual gases. Given constant air pressure and increasing air humidity, the O2 partial pressure and O2 volume concentration accordingly decrease (thinning effect). For a precise thrust comparison, the functional dependence of the expected O2 value on the air humidity is therefore to be taken into account. German patent document DE 10 2006 011 722 B3 describes a possibility for how the humidity of the ambient air can be taken into account when determining the comparison factor. The comparison factor is then part of the proportionality factor for dry fresh air. From the comparison factor for dry air and an air humidity factor, the comparison factor for a particular air humidity level can then in turn be calculated, according to German patent document DE 10 2006 011 722 B3.
However, there are also engine control device functions in which the O2 measurement value itself is not processed, but rather only the lambda value determined therefrom. In these cases, it is insignificant that the air humidity influences the thrust comparison, as long as the multiplicatively falsified O2 concentration is converted into a correct lambda value using the appropriate humidity-dependent equations (1) and (2).